Orthopedic joint replacement surgery may involve arthroplasty of a knee, hip, or other joint (e.g., shoulder, elbow, wrist, ankle, finger, etc.). During joint replacement surgery, a surgeon typically removes diseased bone from the joint and replaces the resected bone with prosthetic implant components. Challenges of joint replacement surgery include determining the appropriate position for implant components within the joint relative to the bone and other implant components and accurately cutting and reshaping bone to precisely correspond to the planned placement of the implant components. Inaccurate positioning of implants may compromise joint performance and reduce implant life.
A surgical system for joint replacement surgery can include a haptic device configured to be manipulated by a surgeon to guide a surgical cutting tool to perform a procedure on a patient. For example, a surgeon can manipulate the haptic device to sculpt a bone so that an implant component can be installed on the sculpted bone. Prior to surgery, a three dimensional model of the bone is created using software techniques. The software model is used to generate a surgical plan, that includes, for example, resecting bone (e.g., using the surgical cutting tool) and inserting implant components. During surgery, the surgeon manipulates the haptic device to move the surgical tool to cut bone, and the haptic device provides force feedback to prevent the surgeon from moving the surgical tool in a way that does not conform with the surgical plan. For example, if the surgeon's movement of the haptic device would cause the surgical tool to resect too much of the patient's bone, the haptic device can apply resistance against the surgeon's movement to prevent the resection. A navigation or tracking system can be used to determine a pose (i.e., position and/or orientation) of the bone, the haptic device, the surgical tool, and/or other objects of interest. As is well known, pose data from the tracking system can be used for registration and real-time object tracking.
For example, U.S. patent application Ser. No. 11/357,197 (U.S. Pub. No. 2006/0142657), which is hereby incorporated by reference herein in its entirety, describes that objects in physical space (e.g., anatomy, surgical tools, etc.) may be registered to any suitable coordinate system, such as a coordinate system being used by a process running on a computer associated with a surgical system. For example, utilizing object pose data captured by a tracking system, the surgical system can associate the physical anatomy and the surgical tool with a representation of the anatomy (such as a computer-generated three-dimensional model or image of the anatomy). Based on the tracked object and registration data, the surgical system can determine, for example, (a) a spatial relationship between the image of the anatomy and the relevant physical anatomy and (b) a spatial relationship between the relevant physical anatomy and the surgical tool so that the computing system can superimpose (and continually update) a virtual representation of the tool on the image of the anatomy, where the relationship between the virtual representation of the tool and the image of the anatomy is substantially identical to the relationship between the actual surgical tool and the physical anatomy. Additionally, by tracking not only the tool but also the relevant anatomy, the surgical system can compensate for movement of the relevant anatomy during the surgical procedure (e.g., by adjusting a virtual object that defines a surgical cutting boundary in response to the detected movement of the anatomy).
The tracking system enables the surgical system to determine (or track) in real-time a pose of tracked objects, such as the bone. One common type of tracking system is an optical tracking system that includes an optical camera configured to locate in a predefined coordinate space specially recognizable markers (e.g., LEDs or reflective spheres) that are attached to the tracked object. However, optical tracking systems require a direct line of sight between the optical camera and the markers. This restricts the movement of the surgeon during surgery because the surgeon cannot interfere with the optical communication between the optical camera and the markers. As a result, the surgeon's movement is limited not only by the location of physical equipment in the operating room but also by lines of sight between the optical camera and markers. Further, other unavoidable surgical side-effects can interfere with the optical communication, such as bone debris that is generated during a bone resection and occludes the surface of one or more markers. Additionally, while optical tracking systems are often accurate, they can be cost-prohibitive.
Another type of tracking system is an inertial tracking system, which uses an inertial measurement unit (IMU) to track an object. An IMU is an electronic device that includes a combination of accelerometers and/or gyroscopes to measure characteristics of an object, such as the object's velocity, orientation, and/or gravitational forces. For example, an IMU can measure three degrees of freedom of the acceleration and three degrees of freedom of the angular rate of the IMU. Using these measurements, the inertial tracking system can estimate the current six degree of freedom pose of the IMU based on a previously determined IMU pose (e.g., an initial (or starting) reference pose) and changes in acceleration and angular rate of the IMU over time. While inertial tracking systems can be more cost-effective than other tracking systems, in general inertial tracking systems introduce greater error through drift. That is, since new poses are calculated from previously determined poses and measured changes in acceleration and angular rate (without reference to any external references), the errors of the tracking process are cumulative such that the error in each new estimated IMU pose grows with time. Specifically, the inertial tracking system integrates the linear accelerations and angular velocities provided by the IMU to calculate the new IMU pose (the acceleration data is often double integrated). The accumulated error leads to “drift,” or an ever-increasing difference between where the inertial tracking system thinks the IMU is located and the actual IMU pose. If the drift is not compensated for, the pose of the tracked object (e.g., the bone) can be incorrectly predicted based on the difference between the predicted IMU pose and the actual IMU pose. This could cause the surgical system to be improperly configured unbeknownst to the surgeon, which could lead to the surgical plan being carried out improperly (e.g., performing improper resections).
In view of the foregoing, a need exists for methods and devices which can overcome the aforementioned problems so as to enable computer assisted surgery (CAS) to be carried out when drift occurs between an IMU's calculated pose and actual pose by resetting a reference pose of the IMU.